The U.S. Embassy on Friday advised Americans to avoid drinking in Tokyo's Roppongi nightlife district, warning that some customers have fallen unconscious and been robbed after their drinks were spiked.
It was the second such alert in four months about Roppongi bars.
"The U.S. Embassy continues to receive reliable reports of U.S. citizens being drugged in Roppongi-area bars," the embassy said in statement.
Tokyo is among the safest big cities in the world, but the embassy has reported a rise in incidents of American customers being rendered unconscious or extremely sleepy. Victims awake hours later to find credit cards missing or fraudulently charged for big amounts.
"These cases are very hard to investigate," said Masahito Fujita, vice head of the Azabu police station overseeing Roppongi. "It's difficult to know whether people were just drinking too much or if they were actually drugged."
Canada, Australia and Britain have also warned their citizens to beware.
Canada says in a travel report on Japan that drinks should "never be left unattended."
Roppongi became a nightspot for foreigners shortly after World War II when the U.S. military was posted nearby.
It remains popular with tourists and Western expatriates drawn to its bars, lounges and dance floors.